1. Field of Invention
This invention relates to an apparatus for forming an optical membrane of a uniform thickness, such as a reflection coating and an antireflection coating, on a curved surface of an optical device (e.g., a lens, a concave or convex mirror).
2. Description of the Related Art
With a conventional optical membrane forming apparatus, the particles vaporized by the vaporization source J are spattered toward the optical device K held above the vaporization source J, and deposited on the surface of the optical device K, as shown in FIG. 6. If the vaporization source J is placed on a plane, the thickness of the membrane formed on the optical device K in the ideal state is defined by equation: EQU t.infin.cos.sup.2 .theta./d.sup.2 (1)
where t is the thickness of the membrane formed on the optical device K, .theta. is the angle between the vertical axis extending upward from the vaporization plane and the spattering direction of the vaporized particle, and d is the distance between the vaporization source J and the optical device K which is positioned in a plane parallel to the plane on which the vaporization source J is placed.
As is defined by equation (1), the thickness of the membrane formed on a plane which is parallel to the vaporization plane with a certain distance from the vaporization plane is the greatest at a point directly above the vaporization source J, and decreases as .theta. increases (that is, the position on the plane is apart from the vaporization source J). For this reason, the thickness of the membrane on the optical device K is apt to be uneven. In addition, the actual distribution of vaporization is likely to deviate from the ideal state depending on the type and shape of the sample to be vaporized, the atmosphere of the vacuum chamber, and the vaporization conditions. Especially when forming a membrane on an optical device having a curved surface, such as a concave mirror and a convex mirror, the degree of unevenness in the thickness of the optical membrane becomes large.
In order to correct the unevenness, an apparatus shown in FIG. 7 was proposed. The apparatus 101 for forming a uniform optical membrane comprises a vacuum chamber 107, in which a substrate holder 2 which rotates about its axis is held above the vaporization source 110. Lens substrates 3 are mounted on the substrate holder 2. Each of the lens substrates 3 is also rotational about its axis, which they are revolved about the rotational axis of the substrate holder 2. A mask 106 is positioned between the vaporization source 110 and the lenses 3. A sample is heated in the vaporization source 110, and the vaporized particles are deposited on the lens substrates 3, whereby a membrane is formed on each of the lens substrates 3.
The substrate holder 2 holds a plurality of lens substrates 3 so that each of the lens substrates 3 rotates about its axis, as shown in FIG. 8. The round aperture 8 in the center of the substrate holder 2 is a monitor aperture, in which a quartz oscillator is fixed. The quartz oscillator detects any changes in the thickness of the membrane formed on the lens substrate 3 as changes in the frequency of the quartz oscillator itself.
The mask 106 is provided for the purpose of adjusting the amount of vaporized particles that reach the region in which the membrane tends to be thicker than in other regions. In this sense, the mask 106 functions as a correction plate. This mask 106 restricts the amount of the vaporized particles incident to the lens substrate 3 at a normal angle (that is, the particles striking the center of the lens substrate 3), while it only slightly adjust the amount of the particle incident obliquely to the lens substrate 3 (that is, the particles striking the periphery of the lens substrate 3), such that the thickness of the membrane becomes uniform on the lens 3.
In general, an optical membrane has a multi-layer structure which consists of two or more materials. When forming such a multi-layer membrane on the lens substrate 3 using a conventional optical membrane forming apparatus 101, the vaporization sample in the vaporization source 110 is changed, while maintaining the vacuum in the chamber 107.
Because several different vaporization materials are used to form a multi-layer membrane, each layer having a similar thickness, it is desirable to use a mask 106 suitable to each material (or each layer). However, since it is difficult to change the mask 106 each time the vaporization material or the sample is change, while maintaining the vacuum level constant, only a single mask 106 which is designed so as to be suitable to the vaporization material which is the most dominant (i.e., which forms the thickest layer) in the membrane is used in the conventional technique. In other words, the mask 106 takes into account the vaporization distribution of the most dominant layer in the membrane.
With a single mask 106, the thickness of each layer of the multi-layer membrane may vary depending on the position on the lens substrate 3, and as a result, the designed optical properties may not be achieved.
For example, if a three-layer anti-reflection membrane having a MgF.sub.2 layer, a ZrO.sub.2 layer, and an Al.sub.2 O.sub.3 layer in this order from the top, with the thickness of the quarter wavelength, the half wavelength, and the quarter wavelength, respectively, is formed on a lens with the ratio of the radius of curvature R to the effective diameter D is 1.4:2, a designed thickness of the membrane is achieved at and near the vertex of the lens, but the thickness of the membrane deviate from the designed value at the periphery of the lens.
The reflectance of such an anti-reflection membrane formed by the conventional apparatus 101 is shown in FIGS. 10 and 11. FIG. 11 shows the reflectance at the vertex of the lens, and FIG. 11 shows the reflectance at the periphery of the lens. As is shown in these figures, the reflectance of the lens varies between the vertex and the periphery because the thickness of the membrane deviates from the designed value at the periphery of the lens.